The present invention relates to a semiconductor device and a method of fabricating the same.
Conventionally, a NAND flash memory has been developed as a nonvolatile semiconductor memory. A memory cell transistor of this NAND flash memory has a structure in which a floating gate electrode formed on a semiconductor substrate via a tunnel insulating film and a control gate electrode formed on this floating gate electrode via an inter-electrode insulating film are stacked.
Recently, to decrease the cell size, a method of using an alumina (Al2O3) film, instead of the conventional ONO film (a stacked film in which a silicon oxide film, silicon nitride film, and silicon oxide film are stacked), as an inter-electrode insulating film is proposed (reference 1).
Since the alumina (Al2O3) film is a high-dielectric-constant film having a relative dielectric constant higher than that of the ONO film, the area of the inter-electrode insulating film can be reduced. As a consequence, the cell size can be decreased.
The NAND flash memory stores “1” data in the memory cell transistor by discharging electrons from the floating gate electrode to the semiconductor substrate, and stores “0” data in the memory cell transistor by injecting electrons into the floating gate electrode from the semiconductor substrate.
Unfortunately, the density of a high-dielectric-constant film is low. Therefore, if a high-dielectric-constant film is used as the inter-electrode insulating film, electrons injected into the floating gate electrode from the semiconductor substrate by applying an electric field of a predetermined level between the control gate electrode and semiconductor substrate penetrate through the inter-electrode insulating film. This increases a leakage current flowing through the control gate electrode.
To prevent this, therefore, it is necessary to suppress the leakage current by performing a predetermined heating process (annealing) for a high-dielectric-constant film deposited on a conductive layer serving as the floating gate electrode, thereby modifying the high-dielectric-constant film.
The reference concerning the use of the alumina (Al2O3) film as the inter-electrode insulating film is as follows.    Reference 1: Symposium on VLSI Technology Digest of Technical Papers, p. 117, 1997